Estimating the irradiance spectrum from measurements in a limited number of spectral bands

Accurate measurement and characterisation of fluctuations in the irradiance environment is important for many areas of optical remote sensing. This paper describes a method of estimating spectral irradiance over the region 400 – 1000 nm from the radiance of a calibrated reference panel, measured in four narrow spectral bands (FWHM approx.10 nm). The reproducibility of the method was found to have an average root mean squared error of approximately 30 mWm-2nm-1 over the region 400 nm to 1000 nm when applied to spectra covering a range of clear sky conditions typical of mid-latitude temperate regions

Estimating spectral irradiance from measurements in seven spectral bands

Accurate measurement and characterisation of fluctuations in the irradiance environment is important for many areas of optical remote sensing. This paper reports a method of estimating spectral irradiance over the VNIR region (400 - 1100nm) from the radiance of a calibrated reference panel, measured in seven narrow (10nm) spectral bands. Earlier work established the potential for estimating spectral irradiance from multi-band data using a neural network technique (Milton et al., 2000). The approach described here uses linear regression analysis to regenerate the irradiance spectrum from data in seven reference wavelengths. The method was tested using data from a specially designed multiband radiometer – the INdependent SPectral IRradiance Estimator (INSPIRE). The irradiance spectrum was partitioned into a number of distinct regions within each of which the spectral irradiance was estimated from irradiance measured at one of the reference wavelengths. The precision of the method was found to be better than ±5% over most wavelengths from 400nm to 1100nm. Furthermore, the slope coefficients of the individual regression models were found to be sensitive to the sky radiance conditions, especially over the region 600-760nm, and improvement in the precision of the predicted spectrum (to within ±3%) was obtained by taking the diffuse-to-global (D:G) irradiance ratio at the time of measurement into account

The temporal dynamics of calibration target reflectance

A field experiment investigated the hypothesis that the nadir reflectance of calibration surface substrates (asphalt and concrete) remains stable over a range of time-scales. Measurable differences in spectral reflectance factors were found over periods as short as 30 minutes. Surface reflectance factors measured using a dual-field-of-view GER1500 spectroradiometer system showed a relationship with the relative proportion of diffuse irradiance, over periods when solar zenith changes were minimal. Reflectance measurements were collected over precise points on the calibration surfaces using a novel mobile spectroradiometer device, and uncertainty in terms of absolute reflectance was calculated as being < 0.05% within the usable range of the instrument (400-1000nm). Multi-date reflectance factors were compared using one-way ANOVA and found to differ significantly (p = 0.001). These findings illustrate the anisotropic nature of calibration surfaces, and place emphasis on the need to minimise the temporal delay in collection of field spectral measurements for vicarious calibration or empirical atmospheric correction purposes

Sources of uncertainty in vicarious calibration: understanding calibration target reflectance

A field experiment investigated the hypothesis that the nadir reflectance of calibration surface substrates (asphalt and concrete) remains stable over a range of time-scales. Measurable differences in spectral reflectance factors were found over periods as short as 30 minutes. Multi-date reflectance measurements were compared using ANOVA and found to differ significantly (p = 0.001). Surface reflectance showed a relationship with the relative proportion of diffuse irradiance, over periods when solar zenith changes were minimal. These findings illustrate the anisotropic nature of calibration surfaces, and place emphasis on the need for collection of diffuse and global irradiance measurements at the time of remotely-sensed data acquisition

Estimating spectral irradiance from a limited number of discrete bands in the visible / near infrared

Accurate measurement and characterisation of fluctuations in the irradiance environment is important in many areas of optical remote sensing. This paper reports a method of estimating spectral irradiance over the VNIR region (400 - 1100nm) from the radiance of a calibrated reference panel, measured in seven narrow (10nm) spectral bands. The irradiance spectrum was partitioned into a number of distinct regions within each of which the spectral irradiance was estimated from irradiance measured at one of the reference wavelengths. The accuracy of the method was found to be better than ±5% over most wavelengths from 400nm to 1100nm

Calibration of dual-beam spectroradiometric data

An experiment to determine the most accurate and repeatable method for generating instrument inter-calibration functions (ICFs) is described, based upon data collected with a dual-beam GER1500 spectroradiometer system. The quality of reflectance data collected using a dual-beam spectroradiometer system is reliant upon accurate inter-calibration of the sensor pairs to take into account differences in their radiant sensitivity and spectral characteristics. A cos-conical field-based method for inter-calibrating dual-beam spectroradiometers was tested alongside laboratory inter-calibration procedures. The field-based method produced the most accurate results when a field-derived ICF collected close in time was used to correct the spectral scan. A regression model to predict the ICF at a range of wavelengths was tested, using inputs of solar zenith angle, cosine of solar zenith angle and broadband diffuse-to-global irradiance ratios. The linear multiple regression model described up to 78% of the variability in ICF; the remainder of the variability was most likely due to complexities of instrumental behaviour in response to warm-up time, ambient temperature and environmental conditions at the time of measurement. Collection of ICFs using a stable laboratory source was shown to provide unsatisfactory results due to differences between lamp outputs and the field-measured solar spectrum. Consequently, the most practical and accurate method of deriving inter-calibration functions is to use field-derived ICFs, collected close in time and space to the data requiring correction

Reflectance panel anisotropy and diffuse radiation - some implications for field spectroscopy

Field goniometer measurements were obtained to examine the angular variation in the reflectance of direct beam, diffuse and global radiation from two types of SpectralonTM panel. The results indicate that optical grade (99% reflective) and grey (20% reflective) SpectralonTM exhibit different non-Lambertian properties with respect to direct beam irradiance. The angular variation in the reflectance of diffuse radiation by the panel appears independent of the panel type but varies with the diffuse to global (D : G) irradiance ratio, especially at large solar zenith angles. The combined effect of the angular response to direct beam and diffuse radiation is that panel reflectance of the global flux shows only slight variation with angle for solar zenith angles up to 55 for optical grade SpectralonTM. For larger solar zenith angles panel reflectance increases markedly with angle